Endoprosthesis with a prosthesis part made of viscoelastic synthetic resin

ABSTRACT

Endoprosthesis with a prosthesis part (4) of viscoelastic synthetic resin such as polyethylene which is intended to be fastened to a support part (1) of more resistant material such as metal by a screw element (3). The screw element has a release locking which consists of a sequence of elevations (14) and depressions which dig into a surface (13) of the synthetic resin with plastic deformation of the latter. For this purpose, a pair of surfaces on the screw element and the synthetic resin part are chosen, whose spacing is determined by the design (in particular by hard stops 8, 9) and which surfaces do not transmit the force with which the screw element is tightened.

With endoprostheses, a part made of viscoelastic synthetic resin such aspolyethylene, which forms, for example, a joint surface of theendoprosthesis, often has to be fixed to a metal support part. Thisscrew requires locking against undesired rotation in the releasedirection. In mechanical engineering, a screw lock is known(EP-A-0290967) which is based on the fact that the force-transmittingend face of the screw is provided with radial ribs which dig into thecountersurface. The depth to which the ribs penetrate into thecountersurface is determined by the force with which the screw istightened. It can be limited by providing deepened contact surfacesbetween the ribs, which contact surfaces, at the intended penetrationdepth of the ribs, bear against the countersurface. However, theparticular penetration depth of the ribs is dependent on the screwingforce. If the screw is loose, the locking also no longer functions. Inthe case underlying the invention, on the other hand, only a slightscrewing force, if any, normally acts on the synthetic resin part to befastened; the known principle can therefore not be directly used here.

The invention therefore aims to find a screw lock which is adapted tothese specific conditions. The solution according to the invention isprovided by an assembly comprising a prosthesis part of viscoelasticsynthetic resin fastened to a support part of the endoprosthesis bymeans of a screw element. The support part is more resistant to plasticdisplacement than the synthetic resin prosthesis part and is providedwith an abutment while the screw element includes a head and a shankwith a stop surface on the shank in spaced relationship to the head. Thestop surface cooperates with the abutment to limit axial movement of thehead relative to the prosthesis part and to seat the screw element. Theseated position of the screw element is determined by means of theinteracting stop surface on the screw element and the abutment on thesupport part. The head of the screw element has saw-toothed elevationsfacing the synthetic resin prosthesis part. The saw-toothed elevationsare formed by tapered flanks forming leading surfaces during tighteningof the screw element and steeper tooth flanks directed against therelease direction of the screw element. In this way, the leadingsurfaces effect a viscoelastic displacement of the synthetic resinduring tightening of the screw element, after which the displacedsynthetic resin volume relaxes at least partly into the depressionsformed between the elevations so that the steeper tooth flanks engagethe relaxed surface of the synthetic resin prosthesis to resistloosening of the screw element.

By virtue of the fact that the relative position of the locking surfaceof the screw element on the one hand and of the surface of the syntheticresin part interacting therewith is determined by the design, thelocking action is independent of the screwing force. It is also ensuredthat the synthetic resin part cannot be damaged by the fact that thelocking surface is tightened with too great an assembly force. Rather,it is ensured under all assembly conditions that the synthetic resinmaterial can only be plastically stressed to a predetermined degree.

The elevations and depressions are designed such that only a plasticmaterial displacement of the synthetic resin occurs, which issubsequently at least partly eliminated again by relaxation. Therelaxation provides a mutual interlocking of the locking surface withthe associated synthetic resin surface, and thus the desired mutuallocking action between the screw element and synthetic resin. Theelevations or depressions of the screw element are preferably designedso as to be saw-toothed in the circumferential direction, their steepertooth flanks being directed against the release direction.

If a screw is used for fastening, it may have a thread at its end, whichinteracts with a threaded bore in the metal support part, and may alsohave, adjacent to said thread at the head end, a thickened shank whichforms a stop on the metal support part, so that, when the screw is inthe end position, the head is located at a predetermined distance fromthe surface of the metal support part, which distance corresponds to thethickness of the polyethylene part. In this case, most of the screwingforce interacts with the support part and only a small proportion withthe polyethylene part. By this means, the position of the lockingsurface is determined by the design. This screw shape is known per se(U.S. Pat. No. 4,822,366), but not in the context of screw locking.

In another embodiment of the invention, the locking surface ispredetermined by the design by the fact that it is arranged on acircumferential surface and interacts with a correspondingly undersizedbore in the synthetic resin part.

The invention is explained in greater detail below with reference to thedrawing which illustrates the exemplary embodiments and in which:

FIG. 1 shows a longitudinal section through a first embodiment,

FIG. 2 shows a longitudinal section through a second embodiment, and

FIG. 3 shows a plan view of the second embodiment.

The polyethylene part 4 is intended to be fastened on the metal supportpart 1 with threaded bore 2 by means of the screw 3. Since the forceusually occurring during tightening of the screw is greater than theforce which is expected of the polyethylene, a shank 7 of greaterdiameter is arranged between the thread 5 and the head 6 of the screw,which shank forms a stop surface 8 which interacts with a stop surface 9on the support part 1 when the screw has reached the intended endposition.

To receive the screw, the polyethylene part 4 has a bore 10 which isenlarged to receive the screw head 6 at 11. The bottom surface 12 of thescrew head therefore interacts with the bearing surface 13 of thepolyethylene part. Normally, the distance between the bottom surface 12and the stop surface 8 of the screw would be made approximately exactlythe same size as the distance between the bearing surface 13 of thepolyethylene part and the stop surface 9 of the support part. If thestop surfaces 8 and 9 bear against one another under tension todetermine the end position of the screw, the bottom surface 12 of thescrew head 6 would then lie directly on the bearing surface 13 of thepolyethylene part.

In the embodiment according to FIG. 1, however, the bottom surface 12 ofthe screw head, as locking surface, is provided with saw teeth 14 whichextend in the circumferential direction and on the one hand aredelimited by the shallow flanks 15 and on the other hand by the steepflanks 16, these surfaces being located such that the shallow flanks 15lead during tightening of the screw and the steep flanks 16 lead duringrelease of the screw. Furthermore, the distance between the tips of theteeth 14 and the stop surface 8 is less than the distance between thebearing surface 13 and the stop surface 9, so that, during tightening ofthe screw, the saw teeth 14 dig into the compliant polyethylene materialof the bearing surface 13, material being at least partly plasticallydisplaced out of the way of the saw teeth 14. This has the consequencethat, directly after the tightening, only a little synthetic resinmaterial, if any, which might prevent the release of the screw, isdisposed opposite the steep flanks 16. However, since polyethylene andmany other synthetic resins, which are therefore also suitable for thesubject of the invention, are viscoelastic and therefore manifestrelaxation, that is to say the property, after a deformation, to slowlydeform back to a shape which approaches the original shape, syntheticresin gradually penetrates back into the empty spaces between adjacentsaw teeth 14 and thus also, at least partly, fills the space in front ofthe steep flanks 16. The screw can therefore not be released withoutthis synthetic resin material being removed, which, because of thesteepness of these flanks, is possible only with considerable forceapplication. This force application is all the greater, since thedeformation because of the flank steepness cannot take place by simpleplastic displacement, but rather requires destruction of the syntheticresin material. This is fully adequate to provide release locking withrespect to the small forces which are to be expected in the applicationsdescribed.

The height difference between the stop surface 9 of the support part iand the bearing surface 13 of the polyethylene part may be exactly aslarge as the height difference between the stop surface 8 of the screwand the tooth base 17 of the saw teeth, so that the complete saw-toothvolume is available for the displacement of the synthetic resin.However, the height difference between the surfaces 9 and 13 may also besomewhat lower, namely may correspond approximately to the averagedistance between the stop surface 8 and the shallow flank 15 of the sawteeth. In such a case, the material volume displaced by the advancinghalf of the saw teeth corresponds approximately to that volume which isavailable in the upper half of the tooth gaps to receive the displacedsynthetic resin material. The optimum dimensioning may be easilydetermined by experimentation.

The arrangement according to FIGS. 2 and 3, unless otherwise describedbelow, is the same as that according to FIG. 1. The difference consistsin the fact that, not the bottom surface 12 of the screw head 6 is usedfor forming the locking surface, but its circumferential surface 18which interacts with the surface 19 of the bore, surrounding the screwhead 6, in the synthetic resin part 4. The circumferential surface 18 isprovided with saw-tooth elevations 20, whose foremost flank 21, in thescrew-in direction, rises gently to permit plastic displacement of thesurrounding material, whereas the reverse side 22 falls away steeply toresist rotation in the release direction and to destroy the syntheticresin material. In the representation according to FIG. 3, it can beseen that, after the plastic displacement during the screwing-in of thescrew 3, the surface 19 of the synthetic resin material has flowed backinto the depressions formed between the saw teeth 20 and therebyinteracts positively with the rear flanks 22 of the saw teeth.

For the purposes according to the invention, a saw-tooth form isparticularly advantageous. However, it is also suitable to useprojections on the bottom surface of the screw elements which require nogreater force threshold during the release movement than during thetightening of the screw, since even relatively low deformation forcesare very often adequate for locking the screw.

The term screw element includes screws, nuts and, furthermore, allelements which can be brought into contact with a bearing surface by ascrewing movement.

An advantageous secondary effect of the invention consists in the factthat, by virtue of the destruction of the synthetic resin during releaseof the screw, it is reliably possible to prevent impermissible reuse ofa synthetic resin prosthesis part.

I claim:
 1. An endoprosthesis comprising a prosthesis part ofviscoelastic synthetic resin fastened to a support part by a screwelement, the material of the support part being more resistant toplastic displacement than the synthetic resin prosthesis part,characterized in that the support part is provided with an abutment andthe screw element (3) has a head and a shank with a stop surface on theshank in spaced relationship to said head, said stop surface cooperatingwith said abutment to limit axial movement of said head relative to saidprosthesis part and seat the screw element on the support part, saidhead having saw-toothed elevations (14) facing the synthetic resinprosthesis part fastened thereby, said saw-toothed elevations (14)having tapered flanks forming leading surfaces during tightening of thescrew element and steeper tooth flanks (16) which are directed againstthe release direction of the screw element whereby the leading surfaceseffect a viscoelastic displacement of the synthetic resin prosthesispart during tightening of the screw element after which the displacedsynthetic resin volume relaxes at least partly into the depressionsformed between the elevations whereby the steeper tooth flanks engagethe relaxed synthetic resin prosthesis, the seated position of the screwelement being determined by the interacting stop surface (8) on thescrew element and the abutment (9) on the support part (1).
 2. Anendoprosthesis comprising a prosthesis part of viscoelastic syntheticresin fastened to a support part by a screw element, the material of thesupport part being more resistant to plastic displacement than thesynthetic resin prosthesis part, characterized in that the prosthesispart includes a bore having a defining surface, the support part beingprovided with an abutment and the screw element (3) having a head and ashank with a stop surface on the shank in spaced relationship to saidhead, said stop surface cooperating with said abutment to limit axialmovement of said head relative to said prosthesis part, said head havinga circumferential surface (18) with saw-toothed elevations (20) engagingthe bore defining surface of the prosthetic part, said elevations beingprovided with tapered flanks (21) forming leading surfaces duringtightening of the screw element and steeper tooth flanks (22) which aredirected against the release direction of the screw element whereby theleading surfaces effect a viscoelastic displacement of the syntheticresin after which the displaced synthetic resin volume relaxes at leastpartly into the depressions formed between the elevations, the boredefining surface (19) having an original position between the positionof the elevations (20) and the position of the depressions of thecircumferential surface (18).
 3. The endoprosthesis of claim 1 whereinthe viscoelastic synthetic resin is polyethylene.
 4. The endoprosthesisof claim 2 wherein the viscoelastic synthetic resin is polyethylene.